Conventionally, coating substrates with a photocatalytic semiconductor such as titanium dioxide comprises employing various sol-gel processes. For example, the production of titanium dioxide (TiO.sub.2) lyophobic sols comprises: hydrolyzing titanium alkoxide in an aqueous system and peptizing the resulting TiO.sub.2 precipitates with appropriate amounts of acid at 80.degree. C. in an ultrasonic field. Due to the complexity of peptization and the fact that the average particle size in the end product depends upon the relative rates of breakdown and aggregation, significant consideration is given to the pH of the solution, ionic strength, particle concentration, and temperature. Once peptized, the sol to gel transformation occurs via water evaporation from the sol. The gel is then heated to form the crystalline form of the TiO.sub.2. Removal of physically absorbed water and organic solvents occurs at 100.degree. C., burning off of bonded organic groups and nitrate ligands occurs at 200.degree.-350.degree. C., crystallization of TiO.sub.2 occurs at 350.degree.-450.degree. C., and conversion of anatase to rutlie phase occurs at 450.degree.-600.degree. C. (see Physical-Chemical Properties of TiO.sub.2 Membranes Controlled by Sol-Gel Processing, Qunyin Xu and Mark Anderson, published in Multicomponent Ultrafine Microstructure, Mat. Res. Soc. Symp. Proc. Vol. 132, pp. 41-46(1989)). Although sol-gel processes produce stable solutions where the TiO.sub.2 remains in solution during the coating process, these processes are typically time consuming, energy intensive, and laborious.
Another process previously employed to coat substrates with a photocatalytic semiconductor comprised sonicating a slurry of TiO.sub.2 and water to form a suspension, covering the substrate with the suspension, and evaporating the suspension to dryness with a vacuum and heat. Due to the use of the heat, this process can produce a coating with poor adhesion.
What is needed in the art is a simplified method for coating a substrate with TiO.sub.2.